Sense amplifiers are commonly used to compare a measuring current at low value provided by a load (for example, on the order of a few μA) with a reference current. For example, several memory devices (and especially non-volatile memory devices) are provided with sense amplifiers for reading logic values stored in selected memory cells. Typically, each memory device simultaneously reads a word formed by the logic values stored in a selected page of memory cells (for example, 64-256). Therefore, the memory device includes a sense structure formed by a corresponding number of sense amplifiers (each one for reading a corresponding memory cell of the selected page).
The operation of the sense amplifiers typically requires that its terminals receiving the measuring current and the reference current should be kept at a predetermined sensing voltage. For example, in non-volatile memory devices (wherein the logic values stored in the memory cells are typically represented by a threshold voltage of its floating gate MOS transistors), the sensing voltage is used to bias the selected memory cells for reading (so that their MOS transistors are conductive or non-conductive according to the stored logic value).
An accurate control of the sensing voltage is desirable in many applications of the sense amplifiers. For example, in non-volatile memory devices the sensing voltage should be maintained at a value such as to enable the correct discrimination of the logic values stored in the selected memory cells, but without any alteration thereof. This may be particularly important when the sensing voltage has relatively low value (for example, <1-2 V).
For this purpose, the sense amplifiers are usually equipped with a voltage regulator (for regulating the sensing voltage to limit possible variations with respect to its desired value). A typical implementation of such voltage regulator is with transistors (for example, of the MOS type) in cascode configuration. This structure allows pre-loading of the terminals of the sense amplifiers (to the sensing voltage) in a relatively fast way, even when they are connected to a load with high capacitance (for example, a column of memory cells in the non-volatile memory devices). Particularly, in a cascode configuration with fixed control (for example, of the gate type), the sensing voltage is regulated by controlling the transistors of the voltage regulator by a biasing voltage of constant value (provided by a biasing stage being common to all the sense amplifiers).
However, the measuring current may cause noise (variable with their value) on the biasing voltage. In addition, the different sense amplifiers may affect each other (according to the different values of their measuring currents). Therefore, the biasing voltage is normally regulated, for example, by a regulating capacitor being common to all the sense amplifiers that filters the noise caused by them (thereby limiting its effects). Such regulating capacitor has a relatively high capacitance, which increases with the number of sense amplifiers (for example, on the order of 50-150 pF). This involves a considerable waste of area (for example, in a chip of semiconductor material on which the non-volatile memory device is integrated).